1. Field of the Invention
The present invention relates to a protective film for a polarizing filter, a method for producing the protective film from a solution, and the polarizing filter in which the protective film is used.
2. Description Related to the Prior Art
Recently, a technical development of liquid crystal displays used for a personal computer, a mobile, a car navigation system, a television or the like is remarkable, and an amount of their products becomes much larger. It causes to rapidly increase demands and supplies of the polarizing filter which is necessary for the liquid crystal displays and those of a protective film which is used for the polarizing filter. In the polarizing filter, two of the protective films are used as optical films which are adhered to both sides of a polarized film to protect it and to provide stabilization in size. A cellulose acetate film is preferably used as the protective film for the polarization plate, while the cellulose acetate film is excellent in transparency, smoothness, and optical isotropy.
The cellulose acetate film is produced in a film producing method from a solution. Concurrently, cellulose acetate and other additives are dissolved to a solvent for preparing a dope solution (hereinafter, dope). Then the dope is cast on a substrate, such as a endless drum or a band whose surface is smooth, and then dried on the substrate to be a continuous film. The continuous film is peeled from the substrate, and thereafter dried in a drying section having a tension frame and a drying area. In the tension frame, the continuous film is supplied with a specified tension, substantially without contact. In the drying area, the continuous film is fed by rollers and usually exposed in an air blow. Thereafter, the continuous film, as having a length between several hundreds and several thousands meter in accordance with usage thereof and capacity of equipment, is wound around the cylindrical rod which is usually made of resin, metal, woods, thick paper and the like. Then the continuous film is packed as a product.
In producing the continuous film of cellulose acetate, the dope must be enough solidified to have self-supporting properties, and thus it can be peeled as the continuous film from the substrate. However, physical property of the continuous film becomes different in accordance with process before and the situations after being peeled.
There are two methods for peeling the continuous film from the substrate:
(1) drying method, in which a hot air is blown to the dope on the substrate so as to decrease the amount of solvent in the dope to form the continuous film;
(2) cooling method, in which the dope is cooled down on the substrate to gel and to form the continuous film. The above two methods each have merits and demerits. The continuous films produced in the above two methods are sold as the optical film such as the protective film for the polarizing filter.
However, the continuous films produced with the above two methods have different properties from each other. In the continuous film produced with the drying method, the weight percentage of the solvent is decreases below 60% before peeling of the continuous film from the substrate. In this case, as the time of the dry on the substrate is long, there is a difference in chemical structure between the front and back side of the continuous film. However, as the primary dry is advanced slowly, the crystallization of polymer does not proceed so much, and the produced continuous film becomes soft. An elastic modulus of the continuous film in being tensed in the casting direction of the dope is less than about 4 GPa.
In producing the continuous film in the drying method, the dry of the dope proceeds on the substrate. Therefore, when the continuous film is peeled from the substrate, a distribution of the remaining solvent in thickness is large. The density of the remaining solvent in a side of the continuous film that has been contacted to the substrate is larger than in another side. Therefore, the produced film shrinks more on a side of a surface which has been contacted to the substrate than in another side. Thus the continuous film is curled. The curl is decreased in a curl control in situation, as disclosed in Japanese Patent 54-26582.
Otherwise, in the continuous film produced in the cooling method, the percentage of solvent remaining in the continuous film become more than 60%. In this case, the dope has usually no self-maintaining property in a room temperature, but is cooled to become gel and form the continuous film in a gel state. Then the dope is peeled as the film from the substrate, and the film is fed to a tension frame for drying both surface without contact thereon. In this method, as the dope on the substrate is dried in a short time, the difference in chemical structure between the both sides is not so large. Further, as the polymer is crystallized rapidly as being dried from both sides of the continuous film, the speed of drying is high, which causes the produced continuous film hard. The elastic modulus is more than about 4 Gpa. Note that the percentage of solvent remaining in the continuous film is calculated in an absolute drying method. Namely, the weight of a sample of the continuous film is measured, whose value is determined as A (g), and thereafter dried in an atmosphere at 120° C. for 90 minutes, and then measured whose value is determined as B (g).Percentage of remaining solvent (%)=(A−B)/A×100.
By the way, some problems often occur in producing a polarizing filter by combining the optical films having different properties. These problems are especially remarkable when in producing the polarization plate by combining the optical films having different elastic modulus. In FIG. 5A, a flat protective film 81 disposed in a side of a polarized film 82 has a smaller elastic modulus than a protective film 83 disposed in another side of the polarized film 82. As shown in FIG. 5B, the protective film 81, 83 are attached to both sides of the polarized film 82 to construct a polarizing filter 80. Then, while a long time has passed, the polarizing filter 80 has a curl such that the protective film 81 may have smaller elastic modulus, as shown in FIG. 5C.
Further, even though there is no difference in elastic modulus, the difference of thickness between the protective films causes the curl of the polarizing filter, such that the thinner protective film may be disposed in an inner side of the curl. The reason therefor is that a variation in size causes to generate a shrinking force. Thereby the protective films having different rigidities have different resistance forces to the shrinking forces. In the polarized film of the polarizing filter, the rigidity is determined as the product of thickness by elastic modulus. Usually, the polarized film is produced by being extended in a lengthwise after polarizing material, such as the iodine, two color dye, two pigment molecule or the like, is absorbed on a film made of polyvinyl alcohol and the like. Then the protective films are attached through an adhesive agent to both surfaces of the polarized film produced in this method. When a long time passes, then a stress remaining in the polarized film after applied for extending it causes to generate a shrink force in an extending direction in the polarized film. The protective film has a function for supply a force against the shrink force which causes to inhibit the polarizing filter to shrink. However, when there is a difference in the rigidity between the protective films attached to both surfaces of the polarized film, then the amount of shrink is different between the both sides of the polarizing filter. Namely, the surface on which the protective film of smaller rigidity is attached shrinks more than another surface. Accordingly the polarizing filter curls.
Recently, the demand of the polarizing filter accelerately increases, and the more of the protective films are used. In this situation, it becomes more difficult to select the protective films having a small difference in the rigidity between both sides of the polarizing filter. Further, recently, an optical compensation film is attached to the polarized film instead of the protective film. In the optical compensation film, an optical anisotropic layer is formed on a surface of a film body of the protective film. When the optical compensation film is used for the polarizing filter, a birefringence of the liquid crystal has effects for easily perceiving images on a liquid crystal panel even though they are watched diagonally. As the optical compensation film has the optical anisotropic layer, the thickness of the optical compensation film becomes larger. When the protective film is applied to the opposite side to the optical compensation film, then the rigidity of the protective film is smaller than the optical compensation film. In order to make the difference of rigidity between the protective film and the optical compensation film smaller, the thickness of the protective film must be made larger. Accordingly, the liquid crystal display becomes thicker and then the value of thereof as the products will become lower.